Heretofore, the best technique for measurement of alternating current over a wide range of frequencies has been by heating a wire with the alternating current to be measured, using a thermocouple to determine the temperature of the heater wire, and experimentally determining the direct current necessary to produce the same temperature as measured by the same thermocouple. A thermocouple is a pair of junctions of two dissimilar metals, which produces a voltage or electromotive force (emf) dependent upon the relative temperatures of the junctions.
In particular, the most accurate such prior art devices have employed a heater wire and a thermocouple junction enclosed in a vacuum or partial vacuum, with the thermocouple junction thermally contacting (but electrically insulated from) the center of the heater wire. The current in the thermocouple circuit, caused by the unknown alternating current in the heater wire, is determined using a galvanometer. Then, a known quantity of direct current is applied to the heater wire and that current is adjusted until the galvanometer indicates the same temperature has been stabily attained. For reasons of greater accuracy, which are explained below, the direct current step is performed twice; once with the current flowing in each direction through the heater wire. The two direct current measurements are then averaged to produce the rms value of the unknown alternating current.
The fact that a circuit of two different metals, with one of two junctions at a different temperature from the other, will produce a voltage is known as the thermoelectric effect or Seebeck effect, after the German physicist Thomas J. Seebeck. The inverse of the Seebeck effect is the Peltier effect, discovered by the French physicist Jean C. A. Peltier. He discovered that when a current is established in a circuit of two different metals in series, one junction between the two metals will be heated and the other will be cooled. The effect of particular junction heating or cooling depends upon the direction of the current. The extent of heating or cooling energy for a given current depends upon the metals used. An analysis of these effects by the British physicist, William Thompson, later Lord Kelvin, lead to the prediction that a voltage must exist between different parts of the same metal if they are at different temperatures. He demonstrated that, in most metals, if a uniform metal bar is heated at the middle and a current sent from end to end from an external source, the heat would be conducted unequally along the two halves. In a copper bar, for example, the end where the current passes from a colder to a hotter part will be cooler than if no current were applied, and the end where the current passes from a hotter to a cooler part will be warmer than if no current were applied.
The Seebeck or thermoelectric effect is employed in the measuring devices of the prior art described at the outset, in order to produce an electric current representative of the heating effect of the unknown alternating current upon the heater wire. Unfortunately, however, the other two effects mentioned above also affect those devices. In such prior art devices, the heater wire is a relatively high resistance wire, such as a Nickel Chromium alloy, which is connected to leads of a relatively low resistance, such as Dumet. The junctions between the heater wire and its connecting leads are subject to the Peltier effect, meaning that one end of the heater wire is heated and the other end is cooled by the flow of direct current through the wire. (A symmetric alternating current would cancel its own Peltier effect). The adverse effects of the Peltier effect have been reduced in the design of such devices by centering of the thermocouple on the heater wire. Although centering of the thermocouple junction reduces the adverse influence of the Peltier effect at either end of the heater wire, the thermocouple junction is seldom located sufficiently accurately to eliminate all such influence. A high temperature coefficient of resistivity wire is often used for the heater wire in such devices, because it tends to compensate for centering error.
Even if the thermocouple junction were perfectly centered on the heater wire between its connecting leads, however, it would be in the wrong place for elimination of the Thompson effect, as the thermal center for cancellation of the net adverse influence of the Peltier and Thompson effects is not the geometric center of the heater wire. For this reason, the known direct current used for comparison is applied in both directions in making the most accurate measurements with these devices. (Alternating current is not conventionally used for comparison in high accuracy testing because AC standards are not as accurate as DC standards). The additional time required for two direct current measurements makes this AC to DC transfer technique cumbersome, a matter which becomes particularly significant when the techniques are applied to automated measurement equipment. The time delay between measurements also permits introduction of further error, due to changes in ambient temperature.
Further background formation regarding thermocouples in electrical measurement will be found in F. W. Sears & M. W. Zemansky, University Physics, pp. 258-259, 546-553 (2nd Ed. 1955); F. E. Terman, Radio Engineer's Handbook, pp. 926-929 (3rd Ed. 1943); and E. Hausmann & E. P. Slack, Physics, pp. 489-493 (2nd Ed. 1939). In the U.S. patent literature, more background information can be found in Pat. Nos. 3,689,824 (Malcolm), 3,668,521 (Aslan), 3,609,541 (Scott), 3,597,685 (Ford), 3,512,086 (Uiga), 3,267,376 (Harries), and 2,365,207 (Moles).